Prostate cancer skeletal metastases are considered osteoblastic; however, histopathological examination usually reveals underlying osteoclastic activity. A key molecule required for induction of osteoclastic activity is receptor activator of NFkB ligand (RANKL). We have determined that prostate cancer cells express increasing levels of RANKL and induce osteoclastogenesis as they progress to bone metastases. Furthermore, in a murine model, we have demonstrated the ability to inhibit establishment of prostate cancer in bone by blocking RANKL-induced osteoclastic activity. Thus, our hypothesis is that RANKL contributes to the development of prostate cancer skeletal metastases, which we will test in the following specific aims: Aim 1. Identify the mechanisms through which RANKL expression is regulated in prostate cancer cells. To understand why RANKL mRNA expression is increased in prostate cancer skeletal metastases, we will characterize the RANKL gene promoter in vitro and in vivo and determine cis-acting sites and trans-acting factors that induce RANKL expression in skeletal metastases. Aim 2. Determine if RANKL is required for the establishment and progression of prostate caneer skeletal metastases in vivo. We will inject prostate cancer tumors into human bone implanted in SCID mice and administer inhibitors of RANKL (i.e., soluble RANK or anti-RANKL antibody) at the time of tumor injection or after tumors become established. We will determine establishment/progression of tumors and expression of bone remodeling markers in the mice. Aim 3. Investigate the effect of docetaxel and estramustine (DE), and a bisphosphonate (Zometa(R)) (Z) on systemic markers of bone remodeling and RANKL expression in bone metastases in men with prostate cancer bone metastases. Men with scan positive bone metastases will be treated with one cycle of Z or DE. This will be followed by two additional cycles of ZED together. Bone markers and indices of prostate cancer response/progression (PSA/bone scans) will be measured at various timepoints. Pre- and post-treatment biopsies of bone metastases in selected patients will be evaluated for RANKL and OPG protein expression.